This invention relates to the process for surface hardening of a steel workpiece by progressively traversing its length with an inductor coil and immediately juxtaposed thereto, a quench spray device. The surface layers of the workpiece are heated to about 1,000.degree. C. and upon quenching, are transformed to the martensitic phase. Martensite provides high surface hardness and enhanced resistance to fatigue. In an alternative version of the process the steel workpiece is sometimes totally immersed in a bath of quenching fluid. The heat generated by the induction coil vapourises the fluid in the localised region of heating, thereby preventing quenching until the inductor coil has traversed away from this region, upon which the quenching fluid makes contact with the heated region of the workpiece and hardening occurs.
The process is frequently referred to as "Scanning Induction Hardening". The inductor coil is typically a single turn (or part of a turn) of heavy section copper conductor surrounding the work, incorporating water passages for cooling, and is supplied by a low voltage, high frequency, alternating current. Usually, the coil remains stationary, and the workpiece is moved during hardening. It is, however, quite feasible for the coil to be moved and for the workpiece to remain stationary. The taken to refer both to movement of the coil and of the workpiece.
Where the workpiece is circular in cross-section, it is usually rotated between centres so as to distribute the hardening uniformly around the periphery which might otherwise be uneven because of small asymmetries of heating due to coil construction or small irregularities in the quench ring. Such an arrangement is disclosed in U.S. Pat. No. 3,525,842, except that the inductor, in this case, does not surround the workpiece.
According to the invention, a method is provided of straightening a workpiece during such hardening operation whether the bending is in the workpiece initially or results from the hardening operation.
It frequently happens that workpieces bend during induction hardening due to the release of longitudinal stresses resulting from previous straightening operations which have taken place at earlier stages of manufacture of the workpiece or even when the original bar material was manufactured. The induction heating releases only the stresses in the outer layers or "case", so that the residual longitudinal stresses in the core cause the workpiece to bend.
Various methods of overcoming this problem have been devised, for example in U.S. Pat. No. 3,988,179 shafts, or axles, are hardened in a carousel device whereby the workpieces have their outer case layers heated by a single shot inductor at one station and then moved to a second station where a ram or series of rams is brought to bear on the red hot workpiece in order to straighten it prior to quenching. Such an approach has the advantage that it corrects any bending which results from the relieving of stresses in the outer case during the heating phase, but has the serious flaw that it does not provide for further bending which occurs during the quenching of the workpiece. This shortcoming of the prior art is overcome by the present invention, which avoids the application of correcting forces by physical contact between the straightening apparatus and the workpiece. The risk of inducing stress cracks, as with these existing methods, which bend the component is thus substantially eliminated.
Thus during the quench, the martensitic phase material which is formed in the case layers undergoes a volumetric change of about 2 or 3%, so that the outer layer is left in compression and, in consequence, the core material is left in tension. Now if, for any reason, the depth of the hardened layer (typically 1.5 mm for a shaft about 25-30 diameter) is deeper on one side than the other, then the compressive stresses in the case layers will be offset from the tension stresses in the core and so bending occurs. The bending will always be such that the convex side of the workpiece is associated with that side having the thickest martensitic case.